1. Field of the Invention
The present invention pertains to apparatus for selectively sealing a chamber to permit fluid pressure buildup therein. More particularly, the present invention relates to well tools for selectively plugging tubing strings and hydraulically-operated apparatus, such as well packers, to permit the pressure buildup therein necessary for operating such apparatus, and subsequently opening such tubing strings and apparatus without an attendant high pressure surge.
2. Description of Prior Art
In the completion of wells, packers and other devices are anchored and/or sealed to the well casing. Various techniques are used to set, or otherwise operate, such tools, including mechanical or hydraulic actuation techniques. In the latter case, a tool, such as a packer, is lowered into place in the well, and a hydraulic-pressure communicating conduit, leading to the well surface, is established. Then, a hydraulic pressure increase may be effected at the tool by pumping on the fluid in the conduit at the well surface. As the pressure at the tool increases, components of the tool respond to carry out the desired operation, such as the setting of a packer in the well casing. Once such operation is completed, the pressure-communicating conduit may be opened, and further well working processes carried out.
It is a common practice to lower such pressure-actuated tools into position within the well by suspending such a tool from a tubing string extended down the well from the surface. In some instances, the tubing string may be utilized in place after the tool is set, or otherwise actuated. As an alternative, the tubing string may be withdrawn, or replaced with, say, a production string. In any event, the tubing string may serve as a pressure-communicating conduit to actuate the tool. In such case, the tubing string, or an extension thereof, must be sealed closed at or below the position of the tool to be actuated. In many cases, though, the seal must be opened or released after the tool is actuated for continued operations within the well.
To carry out such an operation, an open tubing string, supporting the tool to be placed within the well, is lowered into position within the well. A ball or other sealing device is then dropped down the tubing string and caught on a seat at the bottom of the tool, or in an extension of the tool or tubing string below the tool. Fluid is then pumped into the tubing string at the well surface, thereby building up pressure in the tubing string and in the tool. Appropriate components within the tool move in response to such pressure effecting the setting of the tool or other operation to be carried out.
To open the tool and tubing string again, a common practice includes increasing the fluid pressure within the tubing string beyond that which is required to actuate the tool. Then, a shear pin or similar device is broken to free the seat holding the ball or other plug device. Once this occurs, the ball and seat are free to drop down the well, thereby opening the tool and tubing string as desired.
It will be appreciated that the sudden release of the plug and seat upon the breaking of the shear pin, or such device, in response to the high pressure established within the tubing string is accompanied by a high pressure pulse transmitted down the tubing string beyond the original position of the plug, and into the well and formation below. The shock of such a pulse may be sufficiently great to disturb the underground formation, as well to impart a sharp kick, or vibration, to the tubing string. In the latter case, the tubing string as well as any attached tools, including the just-actuated tool for which the hydraulic pressure buildup was initially introduced, may be dislodged or even damaged. Where the formation itself is disrupted, a decrease in production may result. Aside from these destructive effects which may be caused by such a large pressure shock, the buildup of pressure within the pressure-actuated tool above the pressure value needed to so actuate the tool may itself disrupt the setting of the tool, or even cause damage thereto.
Attempts have been made to solve this problem whereby the pressure within the tubing string may be reduced prior to the release of the seal and seat mechanism. However, these attempts generally require complete equalization of pressure across the seal prior to the release of the seal and seat. Such a complete equalization is impossible where the static fluid pressure head in the tubing string itself is greater than the down hole formation pressure.
U.S. Pat. No. 3,331,378 discloses a plugging device which is placed in a condition to be released after the hyraulic pressure within the tubing string is sufficiently raised to carry out the desired tool setting or other operation. Then, the valve seat holding the ball valve closure remains in plugging configuration as long as the pressure within the tubing string is sufficiently great to press the seat against a locking ring with sufficient force to prevent the locking ring, by friction, from expanding to release the seat. When the tubing string pressure is sufficiently decreased, the reduced frictional force is overcome, and the locking ring expands, thereby releasing the seat and ball valve to open the tubing string.
U.S. Pat. No. 3,090,442 utilizes frictional forces to effect anchoring of a plugging mechanism by a plurality of dogs as the tool-actuating hydraulic pressure is built up. Then, the pressure is sufficiently reduced to lower the frictional forces to a point where they can be overcome by expansion of a compressed spring moving a sleeve to release the anchoring bind of the dogs on the plugging mechanism.